In installations for producing nitrogen under pressure, the nitrogen is usually produced directly at the pressure of use, for example between 5 and 10 bars. Purified air, compressed slightly above this pressure, is distilled so as to produce the nitrogen at the top of the column and the reflux is achieved by expansion of the “oxygen enriched liquid” (liquid at the base of the column formed by air enriched with oxygen) and cooling of the condenser at the top of the column by means of this expanded liquid. The oxygen enriched liquid is thus vaporized at a pressure of between about 3 and 6 bars.
If the size of the installation justifies this, the vaporized oxygen enriched liquid is passed through an expander so as to maintain the installation in the cold state but, often, this refrigerating production is excessive, which corresponds to a loss of energy. In the opposite hypothesis, the cold state is maintained by an addition of liquid nitrogen coming from an exterior source, and the vaporized oxygen enriched liquid is simply expanded in a valve and then travels through the thermal heat exchanger serving to cool the initial air. Consequently, here again, a part of the energy of the vaporized oxygen enriched liquid is lost.
While the invention disclosed in U.S. Pat. No. 4,717,410 (hereinafter referred to as “the Grenier cycle”) is very effective for producing high pressure nitrogen, in order to meet the customer demand for the high-pressure nitrogen product in recent years, even if the Grenier cycle is utilized, boosting product nitrogen by the addition of a nitrogen compressor is often necessary. One alternative is that high pressure nitrogen can be supplied by increasing the top condenser pressure. However this method deteriorates the recovery ratio, as well as the specific power.
In FIG. 2 of the Grenier patent, gas is withdrawn from the lower part of the column and sent to the expander. Because the gas composition is similar to air composition, this means this method deteriorates the nitrogen recovery ratio.
An object of the invention is to provide a process and apparatus to permit the production of high pressure nitrogen with high recovery ratio without an additional nitrogen compressor.